Stormwater runoff treatment substrate and stormwater bioretention system constructed by using substrate

ABSTRACT

A stormwater runoff treatment substrate includes a lower pyrite substrate layer and an upper biochar substrate layer. The lower pyrite substrate layer includes pyrite, oyster shell powder, and sandy materials in a volume ratio of 10:5:85. The upper biochar substrate layer includes biochar or activated carbon, organic nutrient soil, and sandy materials in a volume ratio of 20:3:77.

CROSS-REFERENCE TO RELATED APPLICATIONS

Pursuant to 35 U.S.C. § 119 and the Paris Convention Treaty, thisapplication claims foreign priority to Chinese Patent Application No.202010878892.0 filed Aug. 27, 2020, the contents of which, including anyintervening amendments thereto, are incorporated herein by reference.Inquiries from the public to applicants or assignees concerning thisdocument or the related applications should be directed to: MatthiasScholl P. C., Attn.: Dr. Matthias Scholl Esq., 245 First Street, 18thFloor, Cambridge, Mass. 02142.

BACKGROUND

The disclosure relates to the field of stormwater treatment.Specifically, this disclosure relates to a substrate mixed with biocharand pyrite and a stormwater bioretention system constructed with thosetwo substrates for stormwater runoff treatment.

Urbanization and global climate change greatly increased the imperviousarea of the urban surface in recent years, which leads to a series ofproblems such as urban waterlogging and non-point source pollution.Large amounts of nitrogen, phosphorus, and organic matter are carried bythe excessive runoff into the receiving water, causing a series ofenvironmental problems such as black and odorous water andeutrophication.

To solve those problems, several advanced stormwater management conceptsand systems have been proposed, such as Low Impact Development (LID) inthe United States, sponge cities in China (hereinafter collectivelyreferred to as LID), etc. LID includes many decentralizedinfrastructures, such as permeable pavements, green roofs, constructedwetlands, and bioretention systems. Among them, the bioretention systemhas been widely studied and developed in recent years due to its simplestructure and convenient maintenance. Several new bioretentiontechnologies have been applied in large-scale engineering as wellrecently.

The existing stormwater bioretention system generally consists of sandor soil, with very limited pollutant treatment performance. They canonly effectively remove particulate pollutants such as suspended solids,particulate nitrogen and phosphorus, and particulate organic matter. Fordissolved pollutants such as dissolved phosphate, nitrate, etc., theremoval performance was poor, sometimes even shows net leaching of them.Existing research and technology mainly use two ways to solve the aboveproblems. The first is the construction of the reaction zone. The mostcommon one is to set up an anaerobic submerged zone, that is, to raisethe water outlet of the reactor to increase the area of the anaerobiczone and hydraulic retention time of the facility. This method canenhance the removal of nitrate, and this type of technology has beenextensively studied. The second is to use amended media, which is also ahot spot in future research and technology development, that is, toreplace part or all of the sandy substrate with a certain ratio ofamended media to achieve higher pollutant removal performance. Forexample, using water treatment residue (coagulation residue rich in ironand aluminum) to enhance the removal of phosphates, using woodchip toenhance denitrification, and using vermiculite to improve the ammoniaadsorption.

Although bioretention systems with amended media have achieved higherpollutant reduction than traditional bioretention systems, these amendedmedia still face several problems, such as single function, secondarypollutant leaching, overgrowth of the heterotrophic microbial as well asclogging. For example, the most commonly used woodchip amendment cangenerate organic matter and improve denitrification, however, in-situorganic matter leaching in the early construction stage and thelong-term dry period also exist. Moreover, the labile organic matter isgradually consumed in the long-term operation, which will lead to thedecline of denitrification capacity and even the failure of facilities.In addition, the improvement of woodchips has little effect on theremoval of phosphate and ammonia, and may even cause the production oftwo pollutants due to endogenous decomposition. As for the commonly usedvermiculite and water treatment residues, they can only improve theremoval capacity of one or two specific pollutants, making them hard toadapt to the complex characteristics of stormwater runoff pollutants.Nowadays, few related disclosures can simultaneously and efficientlyachieve comprehensive stormwater pollutants removal.

Pyrite used to be deemed as a waste from the mining industry and a causeof acid mine pollution, but it can be used as an electron donor fordenitrification as well. As an alternative to traditional heterotrophicdenitrification technology, using pyrite as the electron donor forsulfur-based autotrophic technology has achieved several results inwastewater nitrogen treatment. However, pyrite, as a water-insolublemineral, has a relatively low electron supply capacity and can onlymaintain a low rate of denitrification, which increases the hydraulicretention time and volume of wastewater treatment facilities, greatlylimited its further application in wastewater treatment.

SUMMARY

Given the problems and limitations of the above-mentioned stormwaterbioretention system and its substrate, the technical problem to besolved by the disclosure is to provide a stormwater runoff treatmentsubstrate, which can durably improve the removal efficiency of commonpollutants in stormwater runoff and reduce the generation ofby-products. The disclosure also provides a stormwater bioretentionsystem constructed with the substrate.

To solve the above technical problems, the disclosure provides astormwater runoff treatment substrate, which comprises a lower pyritesubstrate layer and an upper biochar substrate layer. The lower pyritesubstrate layer comprises pyrite, oyster shell powder and sandymaterials in a volume ratio of 10:5:85; the upper biochar substratelayer comprises biochar or activated carbon, organic nutrient soil andsandy materials, mixed in a volume ratio of 20:3:77.

In a class of this embodiment, the permeability coefficient of the upperbiochar substrate layer is not less than 200 mm/h, the permeabilitycoefficient of the lower pyrite substrate layer is not less than 300mm/h, and the permeability coefficients of the upper biochar substratelayer and the lower pyrite substrate layer are not higher than 600 mm/h.

In a class of this embodiment, a mixture of 20% biochar, 3% organicnutrient soil, 3% 5-10 mesh quartz sand, 7% 10-20 mesh quartz sand, 40%20-35 mesh quartz sand, 17% 30-60 mesh quartz sand, and 10% 60-120 meshquartz sand by volume make up the upper biochar substrate layer.

In a class of this embodiment, a mixture of 10% pyrite with a particlesize of 1-3mm, 5% oyster shell with sheet length of 1-3mm, 3% 5-10 meshquartz, 2% 10-20 mesh quartz sand, 35% 20-30 mesh quartz sand, 25% 30-60mesh quartz sand, and 20% 60-120 mesh quartz sand by volume make up ofthe lower pyrite substrate layer.

The above-mentioned stormwater runoff treatment substrate canautomatically change the release of organic matter according todifferent stormwater conditions. It can also achieve particle pollutioninterception, ammonium nitrogen and organic matter adsorption andtransformation, phosphate complexion and precipitation, and mixotrophicdenitrification through upper biochar substrate layer and lower pyritesubstrate layer cooperation, thereby improving the removal efficiency ofcommon pollutants in stormwater runoff.

The disclosure provides a stormwater bioretention system constructedwith the substrate, comprising a cell body, a gravel drainage layer, atransition layer, a lower pyrite substrate layer, an upper biocharsubstrate layer, a woodchip protective layer, and a ponding zone in thecell body from bottom to top. A perforated water collection pipe wasinstalled in the gravel drainage layer and connected to the raised wateroutlet pipe. The height of the raised water outlet pipe is equal to thetop height of the lower pyrite substrate layer. An overflow pipe isinstalled at the top of the cell body.

The following advantages are associated with the stormwater runofftreatment substrate of the disclosure:

1. This new substrate has an extensive pollutant treatment range. Theupper biochar substrate layer can effectively adsorb and transformammonia nitrogen, organic matter and other pollutants, and perform acertain degree of heterotrophic denitrification. The lower pyritesubstrate layer can further perform autotrophic denitrification andproduce iron and ferrous ions for phosphorus removal. A small amount oforganic matter in the upper biochar substrate layer will be washed intothe lower pyrite substrate layer and promote the mixotrophicdenitrification as well as slight dissimilated nitrate reduction, whichproduces carbon dioxide and ammonium for the survival of autotrophicmicroorganisms.

2. The stormwater runoff treatment substrate is economical andenvironmentally friendly. Both pyrite and biochar are cost-effectivematerials and can turn waste into treasure. They are suitable for simpleand decentralized passive stormwater treatment systems such asbioretention systems.

3. The stormwater runoff treatment substrate is stable and durable. As amineral material, pyrite has high structural strength and chemicalstability, and will not decompose too quickly or reduce the structuralstability and efficiency of the bioretention system. On the contrary,microbial etching on the surface of the pyrite can increase theroughness of the pyrite. It provides better conditions for theattachment of microorganisms, which can stably facilitate the removal ofnitrogen and phosphorus.

4. Stormwater bioretention system has high denitrification efficiency.Compared with the existing heterotrophic modified bioretention system,the stormwater bioretention system of this disclosure can achieve bothautotrophic and heterotrophic denitrification. Autotrophicdenitrification in the lower pyrite substrate layer plays an importantrole during the drought period, which greatly reduces the need forexternal organic matter addition and restricts the heterotrophicmicroorganisms overgrowth as well as organic matter leaching. Comparedwith the system with solo pyrite amendment, the introduction ofheterotrophic denitrification greatly improves the denitrificationcapacity, and can effectively cope with heavy rainfall events which havehigh load and short hydraulic retention time.

5. The stormwater bioretention system has high utilization efficiency ofcell body volume and fewer by-products. The particle size andpermeability coefficient in the upper biochar substrate layer werestrictly limited to an appropriate range. On the one hand, this will notreduce the water volume reduction performance of the system, on theother hand, it will effectively improve the volumetric water content ofthe upper substrate, expand the area of the anaerobic reaction area,then strengthen denitrification. Besides, the higher volumetric watercontent of the upper substrate can also strengthen the ability of thefacilities to cope with long-term drought and maintain the waterrequired by plants. At the same time, the layout also reduces the entryof dissolved oxygen into the lower pyrite substrate layer, then reducethe generation of sulfate and iron by-products by aerobic decompositionof pyrite.

6. The post-maintenance of the stormwater bioretention system is simple.After the long-term operation, the lower pyrite substrate layer willbecome more and more stable, and the sludge output is small, so there isno need for activation and maintenance; The upper biochar substratelayer may be blocked due to it will intercept a large number ofparticulate pollutants. Meanwhile, after the labile organic matter isconsumed, the organic matter supply capacity of the upper biocharsubstrate layer may decline, resulting in the decline of heterotrophicdenitrification capacity. These two problems can be maintained byretrofitting the surface of the upper biochar substrate layer with newlabile organic matter, which is very convenient.

BRIEF DESCRIPTION OF THE DRAWINGS

The disclosure is described hereinbelow with reference to accompanyingdrawings, in which the sole FIGURE is a structural diagram of astormwater bioretention system of the disclosure.

In the drawings, the following reference numbers are used: 1. Runoffguidance slope; 2. Woodchip protective layer; 3. Upper biochar substratelayer; 4. Lower pyrite substrate layer; 5. Transition layer; 6. Graveldrainage layer; 7. Overflow pipe; 8. Perforated water collection pipe;9. Raised water outlet pipe; 10. Cell body; and 11. Ponding zone.

DETAILED DESCRIPTION

To further illustrate, embodiments detailing a stormwater runofftreatment substrate are described below. It should be noted that thefollowing embodiments are intended to describe and not to limit thedisclosure.

The concept of the disclosure is that compared with wastewater,stormwater has different hydraulic and water quality conditions andphysical and chemical properties. First, the stormwater pollutantconcentration is lower, which can adapt to the trait of the lesselectron supply rate of pyrite system; Second, stormwater treatmentfacilities are usually operated intermittently, which can providesufficient hydraulic retention time in the dry period, to meet thecharacteristics of long retention time for pyrite denitrification;Thirdly, pyrite denitrification is mainly autotrophic denitrification,and its sludge yield is lower, which helps to maintain the goodhydraulic performance of bioretention and meets the needs of lowmaintenance of bioretention facilities; Fourth, pyrite is cost-effectiveand durable, does not need complex material synthesis or processing, andcan turning waste into treasure, which meets the requirements of simplestructure and economic saving of stormwater bioretention system.

The preparation of stormwater runoff treatment substrate of thedisclosure:

The preparation of stormwater runoff treatment substrate has highrequirements. If the ratio is inappropriate, it may lead to severalproblems, such as uneven ratio can lead to facility collapse andparticle leakage; Too small particle size will lead to poor hydraulicconditions and blockage; Too coarse particle size will lead to lowpollutant removal capacity, poor water retention and difficult forplants survival.

The following example provides a substrate ratio considering variousneeds, which can be suitable for common stormwater quality and quantityconditions:

In the upper biochar substrate layer, the volume ratio is: 5-10 meshquartz sand 3%, 10-20 mesh quartz sand 7%, 20-35 mesh quartz sand 40%,30-60 mesh quartz sand 17%, 60-120 mesh quartz sand 10%, powderedbiochar 20% and organic peat soil 3%; The D50 after mixing is about 0.48mm.

In the lower pyrite substrate layer, the volume ratio is: 5-10 meshquartz sand 3%, 10-20 mesh quartz sand 2%, 20-30 mesh quartz sand 35%,30-60 mesh quartz sand 25%, 60-120 mesh quartz sand 20%, 1-3 mm pyrite10% and oyster shell powder 5%; The D50 after mixing is about 0.51 mm.

The quartz sand mentioned above can also be replaced by river sand.

The key to the preparation of stormwater runoff treatment substrate isto select sandy materials with different particle sizes and gradationsin a certain proportion. This certain range of particle sizedistribution will make the permeability coefficient of mixed materialsis into a suitable range. The permeability coefficient is tested by theequal head method. The permeability coefficient of the lower pyritesubstrate layer is not less than 300 mm/h, and the permeabilitycoefficient of the upper biochar substrate layer is not less than 200mm/h, both are not more than 600 mm/h. This range of permeabilitycoefficients can make the bioretention system not only meet the needs ofwater reduction and plant growth but also ensure good pollutantreduction capacity.

As shown in the sole FIGURE, the stormwater bioretention cell of thedisclosure comprises a cell body 10, in which there are gravel drainagelayer 6, transition layer 5, lower pyrite substrate layer 4, upperbiochar substrate layer 3, woodchip protective layer 2, and ponding zone11 from bottom to top. A perforated water collection pipe 8 is installedin the gravel drainage layer 6, and the perforated water collection pipe8 is connected to the raised water outlet pipe 9, the elevation of theraised water outlet pipe 9 is equal to the top height of the lowerpyrite substrate layer 4, and an overflow pipe 7 is installed at the topof the cell body 10.

A runoff guidance slope 1 is set along the edge of cell body 10. Thewoodchips of the woodchip protective layer 2 are 1-2 cm long, and thewoodchip can choose bark. The transition layer 5 is a sand layer with aparticle size larger than the lower pyrite substrate layer and smallerthan the gravel drainage layer, which is used to prevent particles fromthe lower pyrite substrate layer from leaking into the gravel drainagelayer and blocking the perforated water collection pipe.

The purpose of activation at the initial operation of the stormwaterbioretention cells is to accelerate the maturity of the microorganism inthe cells (This step can also be abandoned and make the bioretentionaccept natural rainfall to mature). Specifically, before the firstoperation, a culture medium with tap water or stormwater as solventimmersed in the lower pyrite substrate layer is inversely introducedfrom the raised water outlet pipe 9 to promote the proliferation ofsulfur autotrophic denitrification microorganisms in the lower pyritesubstrate layer. The components of the culture medium are 0.2 g/L KNO₃,0.05 g/L NH₄Cl, 0.5 g/L Na₂S₂O₃·5H₂O and 0.02 g/L KH₂PO₄. After thelong-term operation, when the treatment effect of stormwaterbioretention cell decreases, the maintenance can be applied byrenovating the upper biochar substrate layer, adding organic materialssuch as peat soil, or replacing the top woodchip protective layer usingnew woodchips.

It is assumed that the ratio of the surface area of the stormwaterbioretention cell to the catchment area of the cell is 1:20, and therunoff coefficient is 0.75. When the cell deals with low-intensityrainfall (It is defined as the rainfall within 12 hours is not more than14.9 mm), the stormwater collected in the system enters the cell bodythrough the runoff guidance slope, large particles and suspended solidsare intercepted by the woodchip protective layer, stormwater infiltratesinto the upper biochar substrate layer, and the upper biochar substratelayer adsorbs ammonia nitrogen and organic matter. At the same time, thelower part of the upper biochar substrate layer uses organic matter forheterotrophic denitrification to remove part of nitrate nitrogen. Thenthe stormwater enters the lower pyrite substrate layer which the pyriteis used for autotrophic denitrification to remove nitrogen, and thegenerated iron ions are complexed with dissolved phosphate forphosphorus removal. At this time, because the total flow received by thebioretention cell is less, the hydraulic load is low and the dissolutionof organic matter is less. When the rainfall stopped, the remainingnitrate was removed by autotrophic denitrification in the lower pyritesubstrate layer. In the next rainfall, the treated stormwater in thelower pyrite substrate layer will be replaced by new stormwater, tocontinue the above pollution reduction steps.

When dealing with high-intensity rainfall, the removal process issimilar to that of low-intensity rainfall. However, due to the largerrunoff volume generated during high-intensity rainfall and thepermeability coefficient of the upper biochar substrate layer is in therange of 200-600 mm/h, the stormwater will not permeate immediately, butwill gradually gather in the ponding zone, making the stormwaterbioretention cells operate at full hydraulic load. This will increasethe water head difference, moisture content and infiltration rate of thesystem, strengthen the scouring and organic matter dissolution, greatlyimprove heterotrophic denitrification, and offset the low rate ofnitrogen removal using pyrite only, making the stormwater bioretentioncells still has excellent pollutant removal efficiency under heavystormwater even.

Comparative test

1. Test of Pollutant Leakage at the Beginning of Operation

Compared with the traditional sand bioretention system and the woodchipmodified bioretention system, the stormwater bioretention system of thedisclosure has the advantage of low pollutant leaching. Taking tap wateras the influent, we compared and tested the indicators such as Kjeldahlnitrogen (TKN), nitrate nitrogen (NO₃ ⁻—N), nitrite nitrogen (NO₂ ⁻—N),total nitrogen (TN), total phosphorus (TP), chemical oxygen demand (COD)and ultraviolet absorbance at 254 nm (UV₂₅₄) of the above threebioretention cells. The test results of pollutant leakage after onemonth of initial operation of the cells are shown in Table 1.

TABLE 1 Mean value of pollutant leaching concentration (mg/L) KjeldahlNitrate Bioretention system nitrogen nitrogen TN TP COD UV₂₅₄Bioretention system 0.48 0.43 0.95 0.42 19.4 0.199 of disclosureTraditional sand 0.73 0.90 1.71 0.52 26.7 0.325 bioretention systemWoodchip modified 0.92 0.48 1.43 1.56 65.3 0.595 bioretention system

It can be seen from Table 1 that the pollutant leaching concentration ofthe disclosure is significantly lower than that of the two existingbioretention systems.

Although total iron and sulfate will be generated during pyrite-basedautotrophic denitrification or oxidation, the concentration of these twoby-products is very low in the bioretention system of disclosure, thenet leaching of sulfate is generally not more than 10 mg/L.

Except for the first two operations during the start up phase, the totaliron generation is stably below 0.3 mg/1, which meets the requirementsfor total iron in class III water body of Quality standard forgroundwater of China (GBT-14848-2017).

2. Pollutant Removal Efficiency Test

To test the pollutant removal efficiency, we test the stormwaterbioretention system of the disclosure, the traditional sand bioretentionsystem and the woodchip modified bioretention system using syntheticstormwater.

Assuming that the service area ratio of facilities is 1:20, the runoffcoefficient is 0.75, the rainfall duration is 2 h, and the rainfall is25 mm. 10 large-scale rainfall events are simulated and the pollutantremoval efficiency is calculated. The results of pollutant removal areshown in Table 2.

TABLE 2 Average removal efficiency of simulated runoff pollutant (%)Kjeldahl Nitrate Bioretention system nitrogen nitrogen TN TP CODBioretention system of 85.2 41.7 67.4 80.3 76.3 disclosure Traditionalsand bioretention 63.7 1.8 39.1 45.2 68.5 system Woodchip modified 78.054.6 68.2 16.0 35.6 bioretention system

It can be found that this disclosure can achieve excellent pollutantremoval performance even under heavier rainfall events. Compared withthe traditional sand bioretention system, the pollutant removalperformance of this disclosure was higher. Compared with thewoodchip-modified system, although nitrate removal in the currentdisclosure was slightly lower, the TN removal performance was almostequal, and this new disclosure achieved significantly higher COD and TPremoval performance.

It will be obvious to those skilled in the art that changes andmodifications may be made, and therefore, the aim in the appended claimsis to cover all such changes and modifications.

What is claimed is:
 1. A stormwater runoff treatment substrate,comprising a lower pyrite substrate layer and an upper biochar substratelayer; wherein the lower pyrite substrate layer comprises pyrite, oystershell powder, and sandy materials in a volume ratio of 10:5:85; theupper biochar substrate layer comprises biochar or activated carbon,organic nutrient soil, and sandy materials in a volume ratio of 20:3:77.2. The stormwater runoff treatment substrate of claim 1, wherein apermeability coefficient of the upper biochar substrate layer is notlower than 200 mm/h; a permeability coefficient of the lower pyritesubstrate layer is not lower than 300 mm/h, and the permeabilitycoefficient of each of the upper biochar substrate layer and the lowerpyrite substrate layer is not higher than 600 mm/h.
 3. The stormwaterrunoff treatment substrate of claim 2, wherein a mixture of 20% biochar,3% organic nutrient soil, 3% 5-10 mesh quartz sand, 7% 10-20 mesh quartzsand, 40% 20-35 mesh quartz sand, 17% 30-60 mesh quartz sand, and 10%60-120 mesh quartz sand by volume make up the upper biochar substratelayer.
 4. The stormwater runoff treatment substrate of claim 2, whereina mixture of 10% pyrite with a particle size of 1-3mm, 5% oyster shellwith sheet length of 1-3mm, 3% 5-10 mesh quartz, 2% 10-20 mesh quartzsand, 35% 20-30 mesh quartz sand, 25% 30-60 mesh quartz sand, and 20%60-120 mesh quartz sand by volume make up the lower pyrite substratelayer.
 5. A stormwater bioretention system constructed using thestormwater runoff treatment substrate of claim 1, comprising a cellbody, wherein a gravel drainage layer, a transition layer, the lowerpyrite substrate layer, the upper biochar substrate layer, a woodchipprotective layer, and a ponding zone are successively arranged in thecell body from the bottom to top; a perforated water collection pipe isinstalled on the gravel drainage layer, and the perforated watercollection pipe is connected to a raised water outlet pipe; a height ofthe raised water outlet pipe is equal to a top height of the lowerpyrite substrate layer, and an overflow pipe is installed at a topopening of the cell body.
 6. The stormwater bioretention system of claim5, wherein an edge of an entrance of the cell body is provided with arunoff guidance slope, and a woodchip of the woodchip protective layeris 1-2 cm long.
 7. The stormwater bioretention system of claim 5,wherein the transition layer is a sand layer with a particle size largerthan the lower pyrite substrate layer and smaller than the graveldrainage layer.